would bring the rays nearer together, so that they would fall either less divergent, or parallel on the crystalline humor; and, by being sooner converged to a focus, would fall on the retina.

Mrs. B. Very well, Caroline. This is the reason why elderly people, the humors of whose eyes are decayed by age, are under the necessity of using convex spectacles. And when deprived of that resource, they hold the object at a distance from their eyes, as in fig. 4, in order to bring the focus forwarder.

Caroline. I have often been surprized, when my grandfather reads without his spectacles, to see him hold the book at a considerable distance from his eyes. But I now understand it; for the more distant the object is from the crystalline, the nearer the image will be to it.

Emily. I comprehend the nature of these two opposite defects very well; but I cannot now conceive, how any sight can be perfect: for if the crystalline humor is of a proper degree of convexity, to bring the image of distant objects to a focus on the retina, it will not represent near objects, distinctly; and if, on the contrary, it is adapted to give a clear image of near objects, it will produce a very imperfect one of distant objects.

Mrs. B. Your observation is very good, Emily; and it is true, that every person would be subject to one of these two defects, if we had it not in our power to increase or diminish the convexity of the crystalline humor, and to project it towards, or draw it back from the object, as circumstances require. In a young wellconstructed eye, the two muscles to which the crystalline humor is attached have so perfect a command. over it, that the focus of the rays constautly falls on the

retina, and an image is formed equally distinct both of distant objects and of those which are near.

Caroline. In the eyes of fishes, which are the only eyes I have ever seen separate from the head, the cornea does not protrude, in that part of the eye which is exposed to view.

eye;

Mrs. B. The cornea of the eye of a fish is not more convex than the rest of the ball of the but to supply this deficiency, their crystalline humor is spherical, and refracts the rays so much, that it does not require the assistance of the cornea to bring them to a focus on the retina.

Emily. Pray, what is the reason that we cannot see an object distinctly, if we approach it very near to the eye ?

Mrs. B. Because the rays fall on the crystalline humor too divergent to be refracted to a focus on the retina; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline humor; the rays reach the retina before they are collected to a focus, (fig. 4.) If it were not for this imperfection, we should be able to see and distinguish the parts of objects, which are now invisible to us from their minuteness; for could we approach them very near the eye, their image on the retina would be so much magnified as to render them visible.

Emily. And could there be no contrivance to convey the rays of objects viewed close to the eye, so that they should be refracted to a focus on the retina ?

Mrs. B. The microscope is constructed for this purpose. The single microscope (fig. 5.) consists sim

ply of a convex lens, commonly called a magnifying glass; in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to approach your eye very near the object, for the lens A B, by diminishing the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystalline humor D, by which they are refracted to a focus on the retina, at RR.

Emily. This is a most admirable invention, and nothing can be more simple, for the lens magnifies the object merely by allowing us to bring it nearer to the eye.

Mrs. B. Those lenses, therefore, which have the shortest focus will magnify the object most, because they enable us to bring the object nearest to the eye.

Emily. But a lens, that has the shortest focus, is most bulging or convex; and the protuberance of the lens will prevent the eye from approaching very near to the object.

Mrs. B. This is remedied by making the lens extremely small it may then be spherical without occupying much space, and thus unite the advantages of a short focus, and of allowing the eye to approach the object.

Caroline. We have a microscope at home, which is a much more complicated instrument than that you have described.

Mrs. B. It is a double microscope (fig. 6.), in which you see not the object A B, but a magnified image of it, a b. In this microscope, two lenses are employed, the one, LM, for the purpose of magnifying the object, is called the object glass; the other, N O, acts on the

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